With the spread of CDs and DVDs, development of next-generation optical disks which use blue laser diodes is in progress and demand for larger capacity optical disks is growing. An optical disk device which provides not only a CD readout (reproduction) function but also a readout function for CDs and DVDs and a recording function for CD-R/RW, DVD-RAM, and DVD-R/RW is already commercially available. Therefore, interchangeability should be improved in order to perform recording/readout of optical disks which comply with different standards.
The PRML (Partial Response Maximum Likelihood) method has been widely used as a means to increase the capacity of a magnetic disk because it is very effective in improving S/N ratios. In the PRML method, readout signals at consecutive N times are compared with target signal levels and converted into the most likely bit stream (array). Although the direct slice method has been long used for optical disk readout, it seems no longer able to cope with the growing demand for higher speed and larger capacity optical disks. For this reason, the use of the PRML method for optical disk readout is spreading.
There are two problems in applying the PRML method to optical disk readout. One problem is asymmetry. Since a target signal for PRML is calculated by convolution of impulse response (PR class) and bit stream (array), the voltage level is vertically symmetric with respect to the center value. On the other hand, in an optical disk, when the write (recording) power is increased to increase the asymmetry value, the amplitude of a signal also increases and thus the S/N ratio improves. Therefore, generally in an optical disk, the quality of the signal is the best when asymmetry>0. This phenomenon does not occur in a magnetic disk where the width of the read head is smaller than the mark width. In an optical disk, the readout signal level is asymmetric with respect to the voltage level and thus it is difficult to make it agree with the target signal for PRML.
The second problem is media interchangeability. In conventional optical disks the Direct Slice Method is used for readout and the signal quality is defined as jitter which is a deviation between clock edges. Also in the direct slice method, the DFB (Duty Feed-Back) technique or the like should be used to control the slice level. Since a DFB circuit makes an automatic compensation for asymmetry as mentioned above, it is possible to measure jitter which does not depend on asymmetry.
The Technical Digest of ISOM 2002, 269-271 (2002) (non-patent literature 1) describes, as an example of application of the PRML method to optical disks, Adaptive PRML in which readout is done while the target signal level is adaptively changed to cope with a radial or circumferential disk tilt. FIG. 2 schematically shows the adaptive PRML reproduction method described by non-patent literature 1. A PRML decoder is composed of a Level Error Calculation Unit 12, a Minimum Error Condition Selecting Unit 13, a Compensation Target Value Table 16, and a Level Averaging Unit 17. A readout signal 50 is a signal obtained by conversion of an analog signal into digital data by an A/D converter and its equalization. The level error calculation unit 12 calculates the value of the compensation target value table 16 and the square value (Branch Metric Value) of the readout signal value and the minimum error condition selecting unit 13 selects the most likely bit stream or array and outputs a binary result 51. The level averaging unit 17 recomposes bit streams from the binary result 51, averages them for each signal level, and stores the result in the compensation target value table 16.
The method shown in FIG. 2 provides a solution to the problem of asymmetry (problem 1) because all target signal levels are adjusted in a way to follow readout signals. However, even when a mark is shifted (NG case of large jitter), the target signal level is adjusted to compensate for the readout signal distortion so no error signal is generated and the signal quality may be misjudged as good. This results in a failure to assure media interchangeability (problem 2), making it difficult to configure an optical disk system. As explained above, the conventional PRML method has the problems of asymmetry and media interchangeability.